Remote Access Agent for Caching in a SAN File System

ABSTRACT

A system and method is disclosed for maintaining, in a Storage Area Network (SAN), the consistency of a local copy of a remote file system sub-tree obtained from a remote source. Directory structure of the remote file system sub-tree is mapped to a remote container attached to the SAN and each remote object of the remote file system sub-tree is represented as a local object component of the remote container. Next, each of the local objects are labeled with attributes associated with the represented remote object, and metadata describing each of the local objects is stored in a metadata server. Also, a consistency policy is associated with each of the local objects in the remote container (wherein the policy defines conditions for checking freshness of said labeled attributes), and the local object components of remote container is updated in accordance with the consistency policy.

RELATED APPLICATIONS

This application is a divisional application of pending U.S. applicationSer. No. 10/864,605 filed Jun. 10, 2004.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates generally to the field of file sharing.More specifically, the present invention is related to remote filesystem caching.

2. Discussion of Prior Art

In a Storage Area Network (SAN), a SAN file system is used to providehigh-performance access to large file systems. A SAN file system is atype of cluster file system in which both clients and servers areconnected to a SAN. Sometimes client and server roles are combinedsymmetrically in a single machine, as in the case of GPFS, and sometimesthese functional roles are partitioned onto distinct machines, as in thecase of Storage Tank (ST). Two key characteristics of SANs affectsystems in which all participants are required to have SAN access.First, a SAN has a restricted geographical extent, thus limiting it to arelatively small campus. Second, participating hosts must be trusted notto corrupt file system data.

However, users and applications are often distributed over a large areaor are contained within administrative domains that do not fully trusteach other. In these situations, a bridge is needed to allow filesharing across Wide Area Networks (WAN), administrative domains, orboth. The two ends of the bridge, namely, the import and export sides,generally have different characteristics. The bridge can be constructedat the file-level above a SAN file system or at the block-level belowit. Issues requiring consideration include security, performance, andadministration. A WAN admits a much larger range of trust and hasdifferent threat models than a SAN. Therefore, strong security and aflexible approach to integration and interface with multiple domains arenecessary to ensure data availability and authenticity. Additionalprovisions are necessary to account for performance parameters unique toa WAN, in specific, higher latencies and limited bandwidth.

Issues associated with exporting data from a SAN file system can beaddressed by using an existing distributed file system protocol. Forexample, a node of a General Parallel File System (GPFS) could run aNetwork File System (NFS) or Samba server, which could, in turn, deliverSAN file system data to remote users. These protocols are widelyimplemented, but are not well adapted to a WAN environment. A moresuitable protocol, such as Andrew File System (AFS) or Distributed FileSystem (DFS), could be used. While a DFS server has been integrated,albeit in a limited way, with GPFS on AIX, it is not available for anyother SAN file systems, nor has AFS been ported to such an environment.

A complementary task is importing remote data into a SAN file system sothat local applications have access to it. Machines acting as clients ofa SAN file system (e.g., ST clients or GPFS nodes) can individually rundistributed file system clients, such as NFS or AFS, but forgo many ofthe benefits of a SAN file system. Features missing from these discretesolutions include centralized administration and management, uniformnamespace, shared caching, and reduced WAN usage.

Another approach to circumventing SAN distance limitations is to use anIP-compatible storage protocol such as Internet Small Computer SystemInterface (iSCSI). This gives a SAN file system the capability to expandgeographically and include constituents distributed over a WAN. Issuesassociated with this approach are generally due to performance andscaling. Software for storage systems designed to benefit from lowlatencies of a SAN may require redesign when faced with much larger WANlatencies and an expanded set of more complex failure modes. A SAN filesystem that spans a large geographical area will generally have morenetwork constituents, which can challenge the scaling abilities of afile system's architecture. Specifically, cluster algorithms controllingfile system metadata consistency may behave poorly when the number ofnetwork constituents increases. Additionally, distributed networkconstituents may necessitate the use of security model that is not asrestricted as that of a SAN.

Therefore, there is a need in the art to address data importation from aremote source and data exportation to a remote source. There are atleast two problems with performing these tasks within the context of afile server, specifically, within the same process. First, transferringfile data imposes large network and disk loads on the server, which canseverely impact its performance on other tasks. Second, supportingmultiple protocols necessary to communicate with a variety of remotesources can lead to software compatibility issues, maintenance problems,and large executable footprints.

Existing strategies that address some aspects of this concern bydecoupling metadata handling from the burden of transferring filecontents include File Transfer Protocol (FTP), SAN third-party backup,and HSM using DMAPI. FTP transfers data using distinct control and datachannels. Thus, a separate agent operating at the file-level handlesfile contents. However, because FTP operates at the file-level, it islimited in that it cannot support decoupled metadata and file contentsin a SAN File System.

SAN-based backup systems, such as Veritas™, Pathlight™, andLegato/Celestra™, alleviate server load by providing server-less backup.These systems utilize a SAN environment to copy data between storagedevices, such as disk to tape, using a block-level interface such asSmall Computer System Interface (SCSI). They are limited, however, inthat they do not provide for file system interaction and thus, do notprovide file-level access to data residing on either a SAN or across aWAN.

Hierarchical storage management systems gained additional portabilityand interchangeability with the development of the Data Management API(DMAPI). This interface allows the kernel to intercept references toselected portions of a file system and pass them to a user space DMapplication. Access to data is allowed at the file-level through thefile system, so it does not support a remote DM application nor does ittake advantage of a SAN environment to offload file server machines.

The present invention has wide applicability since it makes limitedassumptions about the architecture of a SAN file system and does notmake extreme performance demands on a WAN.

Whatever the precise merits, features, and advantages of the above citedreferences, none of them achieves or fulfills the purposes of thepresent invention.

DEFINITIONS AND TERMS

WAN (Wide Area Network)—a network characterized by long distances, 25kilometers or more, generally large latencies, often greater than fiftymilliseconds; and relatively low bandwidths, less than ten megabytes persecond.SAN (Storage Area Network)—an environment consisting of hosts, storagedevices, and a high-speed network optimized for data transfer.SAN users—are users or applications running on SAN file system clienthosts.SAN disks—are storage devices attached to a SAN.SAN file system—is characterized by a distributed or clusteredarchitecture in which clients access file contents directly from SANdisks and servers mediate metadata access.storage objects—are a variably-sized abstraction exported byobject-based storage devices providing a higher level interface thanfixed-sized blocks.object-based storage device—allows access to a pool of variably-sizedstorage objects instead of an array of fixed-sized block.attributes—named properties stored with file system objects, aside fromtheir contents.container—a collection of file system objects organized into asingle-rooted hierarchy of directories.directory—a collection of mappings between names and file systemobjects.file—a file system object with attributes whose contents are stored inan array of bytes; contents are sometimes divided logically intofixed-sized pieces called blocks.metadata—a term that refers to all file system data except filecontents, specifically object attributes, content storage attributes,and directory mappings.consistency guarantees—dynamic state information shared between a clientand server regarding file system data copies held by a client.aggregate—a grouping of file system objects, such as members of acontainer or the children of a directory.cache and replica—collections of local copies of remote file data. Acache is a partial set selected and validated by local demand. A replicais a well-defined subset validated asynchronously.data—when used without qualification means file system data, bothmetadata and file contents, generally referring to all the persistentparts of a file system: all the objects, their attributes and contents.object—a file, directory, symbolic link, or other file system residententity which has a user-visible name.remote container—a sub-tree of a remote file system attached to thelocal file system namespace as if it were a locally resident container.attachment—a special file system object that joins the namespaces of twocontainers forming a connection to the root of a target container.distant source—a file system accessible via a WAN.foreign source—a file system accessible via network other than a SANremote source—a repository of file data accessible over a networkreferring collectively to both distant and foreign sources.network file access protocol—a communication protocol for accessing aremote source over a network.network file access client—software implementing the client side of anetwork file access protocol.

SUMMARY OF THE INVENTION

The present invention provides for a method for maintaining, in aStorage Area Network (SAN), the consistency of a local copy of a remotefile system sub-tree obtained from a remote source, wherein the methodcomprises the steps of: mapping directory structure of the remote filesystem sub-tree to a remote container (i.e., a remote container attachedto the SAN), representing each remote object of the remote file systemsub-tree as a local object component of the remote container, labelingeach of the local objects with attributes associated with therepresented remote object, storing metadata describing each of the localobjects in a metadata server; associating a consistency policy with eachof the local objects in the remote container (wherein the policy definesconditions for checking freshness of said labeled attributes), andupdating the local object components of remote container in accordancewith the consistency policy.

The present invention also provides a system for updating, storing, andaccessing, in a Storage Area Network (SAN), a local copy of file systemdata from a remote source, wherein the system comprises: (a) a clientinterface, connected to a SAN client, sending to and receiving from theSAN client, metadata and a request for access to the local copy; (b) aremote container module connected to the client interface; the remotecontainer module managing consistency of the local copy and processingthe request; (c) a cache manager connected to the remote containermodule managing and allocating storage space for the local copy; (d) areplica manager connected to the remote container module; the replicamanager maintaining consistency of the local copy if validation isasynchronous type; (e) a distributed file manager connected to theremote container module; the distributed file manager updating the localcopy by obtaining the remote source file system data; and (f) aconsistency policy maintenance framework connected to the remotecontainer module, the distributed file manager, the replica manager, andthe cache manager, wherein the consistency policy maintenance frameworkstores conditions governing consistency management of the local copy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a general system diagram of the present invention.

FIGS. 2 a-c are data flow diagrams for operations on metadata.

FIGS. 2 d-e are data flow diagrams for operations on content data.

FIGS. 3 a-b, collectively, illustrate data flow in a general systemdiagram of the present invention.

FIG. 4 is a freshness check and fetch process flow diagram.

FIG. 5 is a validation process flow diagram, also illustrating SANclient access to requested content.

FIG. 6 is a process flow diagram illustrating object content importationfrom a remote source, also illustrating SAN client access to requestedcontent.

FIG. 7 is a process flow diagram illustrating object content exportationto a remote source.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While this invention is illustrated and described in a preferredembodiment, the invention may be produced in many differentconfigurations. There is depicted in the drawings, and will herein bedescribed in detail, a preferred embodiment of the invention, with theunderstanding that the present disclosure is to be considered as anexemplification of the principles of the invention and the associatedfunctional specifications for its construction and is not intended tolimit the invention to the embodiment illustrated. Those skilled in theart will envision many other possible variations within the scope of thepresent invention.

FIG. 1 illustrates a general system diagram of the present inventioncomprising MDS 100, SAN client 102, SAN disk 118, remote source 122, andDST 124. DST 124 modifies a SAN file system MDS to store as a remotecontainer, a remote file system directory exported by remote source 122.Stored on SAN disk 118, are caches and replicas, collectively calledlocal copies, of file system objects obtained from a plurality of remotesources 122. DST 124 maintains local copies of remote file systemdirectory and file contents by initializing and updating copies aftermodifications occur. Read requests from SAN client 102 for data in aremote container are received by MDS 100 at client interface 104 and areredirected to DST 124. DST 124 determines if the requested object ispresent, namely available as a local copy in cache or replica storage.If the requested object is present, DST 124 verifies the validity ofrequested object. Verifying the validity of requested object may requirecontacting remote source 122, from which requested data was initiallyobtained. If the requested data is invalid or not present, DST 124fetches the requested file system object from remote source 122.

MDS 100 is further comprised of client interface 104, Replica Manager(RM) 106, RCM 108, MDS Code 110, CM 112, Distributed File Manager (DFM)114, and Consistency Maintenance Framework (CMF) 116. Further comprisingDST 124, outside of MDS 100, is RAA 120. SAN client 102 requests datathrough client interface 104 of MDS 100. Client interface 104 forwardsremote container request to RCM 108. RCM 108 determines whetherrequested object is available as a local copy by checking metadatadescribing the storage of local copies in SAN disks, or acontent-storage attribute. If a requested object is present as a localcopy, its validity is verified in accordance with a consistency policystored in CMF 116. A validity check performed by RCM 108 is comprised oftwo parts, a local check and an optional remote check. A local checkdetermines either the existence of a consistency guarantee or thatremote source 122 has been consulted within a specified time bound.Steps implemented in a local check are obtained from CMF 116. However,if a consistency guarantee does not exist or remote source 122 has notbeen consulted recently, communication with remote source 122 via DFM114 and RAA 120 is initiated. Remote source 122 is contacted todetermine whether a file system object has been modified at remotesource 122, and if it has, a corresponding local copy of the object ismarked as being invalid.

If the requested object is not valid or not present as a local copy, RCM108 forwards an object request to DFM 114 which in turn forwards objectdata request to RAA 120. Thus, DST 124 obtains requested file systemobject from remote source 122. DST 124 component, RAA 120, fetches filesystem object contents and attributes from remote source 122.Furthermore, RAA 120 performs remote source 122 operations comprising:reading contents of a directory at remote source 122, reading objectcontents from a storage location on SAN disk 118, writing to a filesystem object on remote source 122, updating a directory on remotesource 122 by creating or removing an entry, and reading and updatingattributes associated with a file system object on remote source 122.Attributes associated with a fetched remote file system object stored inMDS 100 by RCM 108 and object content is stored directly in a SAN disk118 at a location previously specified in a content-storage attribute inmetadata obtained from RCM 108.

Remote file system objects from remote source 122 are held in a remotecontainer, in which they are organized into a hierarchy with a rootdirectory. A remote container is a local entity of MDS 100 managed byRCM 108. While all remote file system objects from remote source 122 areorganized in a hierarchy and thus related to each other, it is notnecessary for each to exist as a local copy. A remote file server hasthe capability to export multiple sub-trees and hence, be locallyrepresented as multiple remote containers in a SAN file system. Cachedfile system objects and aggregates of related, remote file systemobjects, known as replicas, are stored in remote containers; a singulardifference lies in the local population methods of remote containersholding cache copies versus those holding replicas. RM 106 managesconsistency of file system objects that require asynchronous validation.CM 112 performs storage allocation and bookkeeping functions for RCM108. File system object content is written to and stored in SAN disk 118via RAA 120 without using MDS 100 as an intermediary.

Finally, to return a requested object to SAN client 102, DST 124 returnscontrol to MDS 100, which allows SAN client 102 to access a local copyof requested data from SAN disk 118. Modifications to file systemobjects made locally are updated at remote source 122 in accordance witha policy specified in CMF 116. To preserve consistency of local copiesof remote data with respect to changes made by external clients, SANclient 102 requests a consistency guarantee from MDS 100. MDS 100requests a similar guarantee from DST 124, which in turn obtains aconsistency guarantee from remote source 122. The protocol in which DST124 communicates with remote source 122 determines the format andcontent of a consistency guarantee, for example, a time-basedconsistency guarantee.

The present invention arranges SAN file system contents in containers,specifically remote containers hold DST-managed data originating fromremote sources. Each container comprises one or more directories andeach directory further comprises one or more files, attachment points,and symbolic links. A directory contains subsidiary directories, each ofwhich is identified by a name that is unique within its parentdirectory. The association between a directory name and a file systemobject is known as a mapping. A single directory within each containeris noted as a root directory and is not a subsidiary of any directorywithin a container. A distinguished container provides a system root fora SAN file system. Other directories in a container are present inexactly one other directory in a given container. These uniqueness rulesdo not apply to “.” and “..”, current and parent directories,respectively. An attachment point connects a name in a directory of onecontainer to the root directory of another container. A file systemobject therefore has a pathname, which is a list of directory namesstarting with a file system root and traversing directories andcontainer attachments to the last name on a list identifying a filesystem object. Each file system object also has associated attributes.Of particular interest are file length and content-storage attributes; ablock map or storage object identifier can be obtained from the latter.File system object attributes and directory mappings are collectivelyreferred to as metadata and are stored in MDS 100. Prior to directoperations on file system object contents, operations on metadata occur.FIGS. 2 a-2 c are diagrams illustrating operations utilizing orperformed on metadata by MDS involving DFM 200, RAA 202, and remotesource 204. In the exemplary figures, communication between DFM 200 andRAA 202 occurs via RPC, communication between RAA 202 and remote source204 occurs via a network file access protocol dependant on remote source204, and communication between RAA 202 and SAN disk 206 occurs via SANdisk interface.

In FIG. 2 a, DFM 200 requests a lookup operation by contacting RAA 202.RAA 202 obtains as input from DFM 200 a pathname (e.g., /a/b) to arequested file system object. RAA 202 converts local pathnamerepresentation from DFM 200 to a pathname representation for a remotesource 204. RAA 202 obtains a directory contents listing from remotesource 204 and determines if a requested file system object is present.If RAA 202 determines a requested file system object does exist, ahandle to the requested file system object is returned to DFM 200.

In FIG. 2 b, DFM 200 requests a metadata lookup operation by contactingRAA 202. RAA 202 obtains as input from DFM 200 a file handle, previouslyobtained from a lookup, to a requested file system object. RAA 202provides to remote source 204 a converted remote representation ofpathname from an input file handle and obtains attributes from remotesource 204, for example, the last time the requested file system objectwas modified. RAA 202 converts attributes received (e.g., a timestamp ofthe last modification time) from remote source 204 into a representationcompatible with DFM 200. Lastly, RAA 202 provides to DFM 200 metadatacomprising attributes associated with a requested file system object.

In FIG. 2 c, DFM 200 performs metadata operations resulting inmodification to a remote source 204 object. DFM 200 provides as input toRAA 202 a file handle and an attribute, which is file length in theexemplary figure. RAA 202 converts a file handle provided as input byDFM 200 to a pathname representation for a remote source 204 and passesit along with a file attribute to remote source 204. Remote source 204performs operations on the requested file system object given by RAA 202using as input parameters, attributes provided by RAA 202. Uponcompletion of requested operations, remote source 204 returns to RAA 202a termination status message. RAA 202 forwards termination statusmessage to DFM 200.

In FIGS. 2 d-e, operations on file system object contents are shown. InFIG. 2 d, a fetch operation from remote source 204 is performed, thuswriting remote data to SAN disk 206. In an exemplary embodiment, whereinenvironments utilize storage object devices, DFM 200 provides to RAA 202a file handle, a desired file range, along with an object identifier andan offset. In other embodiments, wherein environments utilize simpleblock storage devices, an object identifier and offset is replaced by arange of block addresses. RAA 202 then queries remote source 204 forfile system object referenced by file handle. Remote source 204 returnsto RAA 202 the requested file system object content and associatedmetadata. RAA 202 passes the requested file system object content and arange of block addresses to SAN disk 206 for storage. SAN disk 206returns to RAA 202 a termination status message. RAA 202 returns to DFM200 metadata associated with the requested file system object including,but not limited to, attributes such as file length and modificationtime.

In FIG. 2 e, SAN disk 206 data is written to remote source 204. DFM 200provides to RAA 202 a file handle to a file system object, a desiredrange within the requested file system object, and a list of blockaddresses indicating where the requested file system object is stored inSAN disk 206. RAA 202 forwards to SAN disk 206 a range of blockaddresses. SAN disk 206 reads data from the provided range of blockaddresses and sends data to RAA 202. RAA 202 provides data from SAN disk206 along with a pathname representation for remote source 204 to remotesource 204. Remote source 204 returns to RAA 202 metadata (i.e., filesystem object attributes) associated with the modification, which inturn returns metadata to DFM 200. In the exemplary embodiment, remotesource 204 is pathname-based (e.g., FTP). In other embodiments, remotesource 204 is handle-based (e.g., NFS).

Collectively shown in FIGS. 3 a and b, is a general system diagramindicating data flow between MDS 300, DST 318, SAN client 320, SAN disk322, remote source 324, as well as within MDS 300 and DST 318. SANclient 320 obtains metadata and a consistency guarantee from MDS 300 viaclient interface 304. SAN client 320 communicates directly with SAN disk322 via SAN disk interface. To access the contents of a file systemobject, SAN client 320 uses a content-storage attribute from metadataspecifying a storage object identifier or block map obtained from clientinterface 304 of MDS 300 and reads from or writes to the specifiedlocation in SAN disk 322. If SAN client 320 performs a read operation onthe identified storage location in SAN disk 322, SAN disk 322 returns toSAN client 320 file contents. SAN client 320 obtains file system dataassociated with retrieved file contents, from MDS 300. A consistencyguarantee provided to SAN client 320 ensures that MDS 300 will providenotification when an object is changed by another SAN client or by aremote source. Consistency guarantees allow a plurality of clients touse file system metadata and file contents simultaneously. Consistencyguarantees from MDS 300 are also used to synchronize clients using byterange and open locks; byte range and open locks are a means ofcoordinating clients reading from and writing to a file.

RCM 308 stores object and content-storage attributes, attributes forvalidating object contents, and directory mappings, collectivelyreferred to as metadata, pertinent to file system objects stored inremote containers. Client interface 304 of MDS 300 passes to RCM 308requests received from SAN client 320 for metadata for a file systemobject, as well as requests for a consistency guarantee or guaranteesassociated with a requested file system object. RCM 308 obtains aconsistency guarantee pertaining to a requested file system object fromDFM 314 and relays said consistency guarantee to client interface 304 ofMDS 300. Thus, MDS 300 is able to extend equivalent guarantees torequesting SAN client 320. Client interface 304 of MDS 300 is also usedby RCM 308 to deliver notification of changes in remote objects receivedfrom DFM 314 back to SAN client 320. In one embodiment, SAN client 320is notified of changes by lock revocation.

CM 312 manages local storage for remote file system object contents. CM312 supports both caching and replication functions. CM 312 implements areplacement algorithm identifying a subset of remote objects that are tobe stored locally. RCM 308 communicates with CM 312 to request storagespace for remote file system objects. CM 312 operates on portions ofremote object content managed by RCM 308, which are either parts offiles, whole files, or entire replicas, depending on a specified levelof granularity. Because RCM 308 contains metadata describing the storageof local copies in SAN disks, and thus, determine local copy presencewithout consulting CM 312, CM 312 also communicates with RCM 308 toindicate the reclamation of storage space and subsequent eviction ofstored remote file system objects. Additionally, CM 312 provides amechanism for attaching to each chunk, pertinent subset of objectattributes. Such a subset of object attributes is used by a replacementalgorithm for determining the best subset to store. A cache replacementalgorithm is influenced by policy input obtained from CMF 316. Forexample, priorities governing competition for cache space in CM 312 areadjusted in accordance with policy input obtained from CMF 316. Actualblock allocation for a storage area governed by CM 312 is handled by MDS300, specifically by server utilities in MDS Code 110, while anobject-based storage system performs block allocation in SAN disk 322.

RM 306 operates in a manner different from that of client interface 104,managing replicas by scheduling the revalidation of their contents.Replicas are well-defined subsets of remote data, which have commonpolicies for validation and update consistency. CMF 316 creates replicasas a result of defining policies on groups, or aggregates, of remoteobjects. Replica management involves the definition of replicas and theprocessing of asynchronous validations for local copies of remote filesystem objects.

DFM 314 communicates with RCM 308 to provide an interface to remotesource 324. DFM 314 provides a method for accessing file system objectsfrom remote source 324 and managing consistency guarantees encapsulatingprotocol-specific details. To implement such a remote data interface,DFM 314 communicates with RAA 326, which implements a protocol necessaryto communicate with remote source 324. DFM 314 interface providesmethods for operations on directories (e.g., lookup and create),obtaining and modifying file system object attributes, obtaining andreturning consistency guarantees, and indirectly reading and writingobject contents. While RCM 308 is not affected by protocol-specificdetails, each implementation of DFM 314 and RAA 326 address the concernsof a single protocol. In one embodiment, RCM 308 connects to a pluralityof DFM 314 implementations further connecting to a plurality of RAA 326implementations. Thus, communication through a plurality of protocolsfor diverse remote sources 324 is enabled.

CMF 316 records administrative preferences for important aspects of DST318 behavior. Namely, preferences are consistency policies forvalidation and update transmission for local copies of remote objects.Validation and update parameters are supplied to DFM 314, whilevalidation type (e.g., asynchronous or synchronous) is supplied to RM306 and RCM 308. For each file system object in a remote container, aconsistency policy is specified and associated. Consistency policies aremaintained in CMF 316 along with entries binding consistency policies tofile system objects. Consistency policies are independent and reusableobjects, while policy entries are keyed by a remote container identifierand a relative pathname of a requested file system object within a keyedremote container. For efficiency of application and specification,consistency policies are defined using a hierarchy of aggregatescomprising directories, sub-trees and remote containers. More detailedspecifications supersede less detailed specifications that occur at ahigher level in a file system hierarchy. Additionally, consistencypolicies are enabled based on file system object attributes, forexample, name and size.

Network file access protocols vary in their provision of support formaintaining the consistency of remote data accessed by a client. Toconceal support for varying levels of consistency maintenance from RCM308, DFM 314 utilizes a mechanism that provides for consistencyguarantees by using delegation and callbacks. A delegation correspondsto a consistency guarantee; it is a request to be notified of certainchanges. A callback corresponds to such a notification. Delegations aredesignated for individual file system objects and for aggregates (e.g.,directories and entire containers). In one embodiment, a delegation istyped and is applied to a specific operation, thus allowing separatecontrol of read and write access to either metadata or object contents.In another embodiment, a delegation has multiple types and applies to aset of operations on either metadata or object contents. Delegation andcallback mechanisms allow network file access protocols providing strongconsistency guarantees to be supported without loss of function bymapping these consistency guarantees, while DFM 314 simulates thebehavior for network file access protocols providing weaker consistencyguarantees. The fidelity of such a simulation is controlled byconsistency policy input from CMF 316. Network file access protocolsalso differ in support for other features, such as file system objectattributes. DFM 314 also encapsulates these differences. The separationof functions between DFM 314 and RAA 326 is protocol and implementationdependent.

RAA 326 is invoked by DFM 314 to contact a requested remote data source324. RAA 326 provides for reading file system object contents andattributes from remote source 324, returning of metadata to DFM 314,writing of file contents to SAN disk 322, and managing authenticationand protocol state for DFM 314. A key feature of RAA 326 interface withDFM 314 is the abstraction of file contents from MDS 300. Rather thanhandling file contents, DFM 314 provides as input to RAA 326 acontent-storage attribute (e.g., storage object identifier and offset,range of block addresses) indicating a location in SAN disk 322 in whichrequested file contents are stored. RAA 326 either writes receivedremote file contents to specified block addresses or reads from blockaddresses on SAN disk 322 locally modified file content. RAA 326performs a read operation when locally modified content from specifiedblock addresses is to be exported to remote source 324 from which itinitially originated.

Consistency Maintenance Process Flow—Freshness Check and Fetching

The task of consistency management is shared between RCM 308, DFM 314,and CMF 316. Referring now to FIG. 4, a process for maintaining theconsistency of local copies of remote data with respect to remote source324 is shown. In step 400, a freshness check of metadata by RCM 308,specifically object attributes, is triggered synchronously, by SANclient 320 access, or asynchronously, by a background RM 306 process. Ifa consistency guarantee exists for a referenced local copy, it isdetermined whether object data content is requested in step 402.Specifically, it is determined whether a SAN client triggered access. Ifin step 402, it is determined that object content is not requested, areferenced local copy is determined to be consistent and the processends with step 404. Otherwise, the process proceeds to step 500.

If a consistency guarantee for a referenced local copy does not exist,attributes corresponding to referenced local copy of remote data arerequested from DFM 314 in step 406. Subsequently, in step 408 remoteobject attributes are fetched from remote source 324. If an existing,revoked consistency guarantee for a referenced local copy isnotification-based, a new consistency guarantee is obtained from remotesource 324 in step 410. Otherwise, if it is time-based, a local timer isset in step 412. A time-based consistency guarantee is automaticallyrevoked when the time interval set by a local timer has passed, whereasa notification-based consistency guarantee remains in effect until it isrevoked upon receipt of an explicit notification from remote source 324.The process proceeds to step 414 where newly fetched attributes aremerged with previous attributes. Merged attributes may cause theinvalidation of directory or file content. For example, if amodification time attribute of a file has changed, associated datacontent is invalidated.

The process returns to step 402, where it is determined whether objectcontents of a referenced local copy have been requested; if such is thecase, object content is validated in step 500. Otherwise, the processterminates with step 404.

Asynchronous and synchronous methods of validation are compatible witheach other and thus, it is possible to specify both methods ofvalidation in a single, hybrid policy. In one embodiment, asynchronous,synchronous, and hybrid policy types specified for different remotecontainers operate simultaneously within a single instantiation of MDS300. For synchronous validation, RCM 308, connected to client interface304, intercepts references to local copies of remote objects andperforms validation before allowing requested data content or attributesto be returned to SAN client 320. When synchronous validation is notspecified, MDS 300 is connected to a local copy through a null remotecontainer module and asynchronous validation occurs as a backgroundprocess, namely, as a background RM 306 process.

Consistency Maintenance Process Flow—Validation

Referring now to FIG. 5, a process for validating local copies of remoteobject content is shown. Before the start of the process, each remotefile system is mapped to a remote container and remote directorystructure of remote file system is mirrored locally. In step 500, localcopies of remote object content are validated by a comparison performedby RCM 308 between current file or directory validation attributes(e.g., modification time, change attribute) with file or directoryattribute labels associated with a referenced object when it was lastretrieved. The process proceeds to step 502 if a comparison indicatesthat object content is invalid. In step 502, remote content importationfrom remote source 324 is initiated. Subsequently, the process proceedsto step 600 to continue on with an object content importation process.If comparison in step 500 reveals valid object content, it is determinedin step 504 whether validation was triggered by SAN client 320 or by RM306. If it is determined in step 504 that SAN client 320 referenced anobject and requested content, RCM 308 returns a content-storageattribute to SAN client 320 in step 506. Subsequently, in step 508, SANclient 320 accesses storage location in SAN disk 322 indicated bycontent-storage attribute. Validation and accessing process terminatesin step 510. If RM 306 triggered validation, the process terminates withstep 510.

Differences between remote directory structure and local containers,such as non-resident files, are rectified upon demand by RCM 308 suchthat objects represented in a container appears the same as SAN-residentdata. If a referenced file system object is invalid or non-resident,referenced file system object content and metadata comprising directorymappings and attributes are obtained from remote source 324.

Consistency Maintenance Process Flow—Object Importation Process

In step 600, the type (e.g., file, directory) of an invalid object ischecked. If it is determined that SAN client 320 has requested a contentstorage attribute (e.g., storage object identifier and offset, range ofblock addresses) for non-resident or invalid file contents, RCM 308 isinvoked to request storage space from CM 312 in step 602. If sufficientstorage space currently exists, step 602 is bypassed; this informationis determined from metadata comprising file length and content-storageattributes. The process continues to step 604 where RAA 326, invoked byDFM 314, retrieves requested file data comprising file contents andassociated attributes from remote source 324. RAA 326 stores retrievedfile contents in SAN disk 322 in a storage location designated by RCM308 in step 606. In step 608, RCM 308 writes the storage location inwhich file content was stored to a content-storage attribute in metadatacorresponding to retrieved file data content. The process continues withstep 610 where retrieved file contents are labeled with current fileattributes, and thus, file content importation portion of the process iscomplete. In step 612 SAN client 320 obtains content-storage attributefrom RCM 308 and accesses valid file content on SAN disk 322. In step626, the process terminates.

Returning to type-checking step 600, if invalid object is of directorytype, the process proceeds to step 614 where a requested directory isretrieved from remote source 324 via DFM 314. Remote source 324 returnsa list of names and attributes for each child object in a directorysub-tree. In step 616, it is determined whether a local entry exists fora name selected from list returned in step 614. If such an entry doesexist, the process proceeds to step 622; otherwise, the process proceedsto step 618 where a local object is created to represent a child objectreferred to by a name chosen in step 616. A local name is alsoassociated with a newly created local object. Continuing to step 620, alocal entry mapping local name to local object is created in a localdirectory for a new object. In step 622, which follows directly afterstep 616 if a local entry for already exists for a selected name, newlycreated local object is labeled with attributes associated with a childobject it represents. The process returns to step 616 and repeats steps616 through 622 for each name in list. After each name in list has beenvisited, directory content importation portion of the process iscomplete. The process continues to step 624 in which SAN client 320accesses a requested, valid directory object in 624, and the processterminates in step 626.

A retrieved file system object is also accompanied by a consistencyguarantee for a requested file system object, or a larger aggregatecontaining a consistency guarantee for a requested file system object.RCM 308 retains a consistency guarantee to ensure the continuity of filesystem object validity. Because file system object metadata is necessaryto represent local storage details of file system object contents, it isnecessary to purge file system object contents from local storage beforecorresponding metadata is purged. DFM 314 is used to request directorymappings and attributes for objects individually (e.g., by looking up asingle pathname) or in bulk (e.g., by reading an entire directory).Results of a request from DFM 314 are converted into a form compatiblewith SAN file system metadata representation by RCM 308 of a dataimportation process.

Consistency Maintenance Process Flow—Object Exportation Process

Referring now to FIG. 7, a process for exporting data to remote source324 is shown. In step 700, modifications made by SAN client 320 aredetected by RCM 308, and local copies of file system objects comprisedof metadata, directory mappings, and file contents are marked in amanner indicating that modifications have been made. Modifications tofile system objects are protected by a consistency guarantee allowingsuch changes.

If modifications to local copy marked in step 700 affect file contents,exportation process continues to step 702. In step 702, RAA 326 obtainsmetadata comprising a content storage attribute and file attributescorresponding to a modified local copy from RCM 308 via DFM 314. In step704, RAA 326 retrieves file content stored in SAN disk 322 at locationindicated by obtained content-storage attribute. RAA 326 exports filecontent and file attributes to remote source 324, as shown in step 706.A schedule determining the frequency and time interval at whichmodification updates are sent to a remote source is dependant on anupdate consistency policy setting. Update consistency policyconfigurations are defined by a delay (i.e. write-behind), which may bezero (i.e. write-through), or by using a method of delegation in which aremote source agrees to notify RCM 308 via RAA 326 and DFM 314, whenremote source 324 requires an update. These considerations control thedelay between the time at which a file system object is modified andwhen modification updates are sent back to a remote source. In oneembodiment, delegation is used to provide single system image semantics.Continuing to step 708, remote source 324 returns remote file attributesand a consistency guarantee to RAA 326, which is subsequently returnedas metadata for storage in MDS 300 by RAA 326 to RCM 308 via DFM 314.File content exportation process terminates in step 718.

If modifications to marked local copy in step 700 affect metadata,directory content exportation process continues to step 710 where RAA326 obtains changed directory entries from RCM 308 via DFM 314 formarked local copies. In step 712, RAA extracts from directory entriesand reformats, a name and a mapping corresponding to a local directory.Following in step 714, RAA 326 exports directory attributes and namechanges obtained in step 712. Proceeding step 716, remote source 324returns remote directory attributes and a consistency guarantee to RAA326, which is returns directory attributes and a consistency guaranteeas metadata for storage in MDS 300 by RAA 326 to RCM 308 via DFM 314.Directory exportation process terminates in step 718.

Remote Access Agent

RAA 326 handles protocol-specific details necessary to managecommunications with a remote source. RAA 326 implements network fileaccess protocols used to communicate with remote source 324 and directstransfer of file content between remote sources and SAN disks. A keyfeature of RAA 326 interface with DFM 314 is that it allows RCM 308 toavoid handling file contents. Instead, DFM 314 passes a content-storageattribute (e.g., storage object identifier and offset, range of blackaddress) obtained from CM 312 to RAA 326. RAA 326 uses block addressesto write to SAN disk 322 contents of remote file system objects itreceives, as well as to read locally modified data from SAN disk 322 andsend it to remote source 324.

RAA 326 code is linked with library code that allows data transfer toand from remote source 324 using a variety of network file accessprotocols. In one embodiment, RAA 326 implementation is specialized forgiven network file access protocols and specific remote sources. Datatransfer is accomplished by making direct calls to network file accessclient code and by making file input/output (I/O) system calls to aremote operating system kernel. RAA 326 is indistinguishable from atypical network file access client from the perspective of remote source324. RAA 326 also utilizes consistency mechanisms supported by networkfile access protocol and maintains these mechanisms in proxy for itscaller, namely DFM 314. RAA 326 also acts as a security proxy for SANclient 320. Depending on the security protocol in use, RAA 326 receivescredentials for a particular user and uses them as necessary to performauthentication. In this manner, authorization to perform a particularfile operation is obtained.

In one embodiment, several agents are instantiated, allowing for animprovement in throughput and parallelism. In another embodiment, anagent is implemented with internal parallelism; it is comprised ofsub-agents implemented on a plurality of distinct machines. Theseparation of the DFM 314 and RAA 326 necessitates inter-processcommunication between machines hosting each module. Because DFM 314interface uses function calls and callbacks, an RPC connection orconnections allowing bi-directional calls are necessary.

Additionally, RAA 326 makes provisions for reading and writing metadataand file contents, as well as managing authentication and protocol statefor DFM 314. RAA 326 acts as a data-handling proxy serving as anintermediary between MDS 300, remote source 324, and SAN disk 322. Aprotocol-specific implementation of RAA 326 is invoked by DFM 314 toinitiate communications with remote source 324. DFM 314 selects animplementation of RAA 326 based on source remote container and RAA 326configuration. DFM 314 sends requests to RAA 326; RAA 326 receivesrequests and identifies appropriate remote host, protocol, and userauthentication for communication. In one embodiment, RAA 326 respondsimmediately by providing a file system object handle for a known path.Otherwise, RAA 326 sends request to remote source 324 and processesreturned response. Depending on network file access protocol, localprotocol state is also managed by RAA 326. Local protocol stateprocessing is comprised of maintenance of: file handle and lock owneridentifiers, sequence numbers for serializing operations, and callbacksfor consistency management.

Additionally, the present invention provides for an article ofmanufacture comprising computer readable program code contained withinimplementing one or more modules to locally store, share, and maintainremote file system data. Furthermore, the present invention includes acomputer program code-based product, which is a storage medium havingprogram code stored therein which can be used to instruct a computer toperform any of the methods associated with the present invention. Thecomputer storage medium includes any of, but is not limited to, thefollowing: CD-ROM, DVD, magnetic tape, optical disc, hard drive, floppydisk, ferroelectric memory, flash memory, ferromagnetic memory, opticalstorage, charge coupled devices, magnetic or optical cards, smart cards,EEPROM, EPROM, RAM, ROM, DRAM, SRAM, SDRAM, or any other appropriatestatic or dynamic memory or data storage devices. Implemented incomputer program code based products are software modules for: (a)storing structured local copies of remote data; (b) storing metadatapertaining to said remote data; (c) validating local copies; (d)accessing local copies utilizing said metadata; (e) importing data froma remote source when changes to remote data occur; and (f) exportingdata to a remote source when changes to local copies occur.

CONCLUSION

A system and method has been shown in the above embodiments for theeffective implementation of a remote access agent for caching in a SANfile system. While various preferred embodiments have been shown anddescribed, it will be understood that there is no intent to limit theinvention by such disclosure, but rather, it is intended to cover allmodifications falling within the spirit and scope of the invention, asdefined in the appended claims. For example, the present inventionshould not be limited by software/program or specific computinghardware.

The above enhancements are implemented in various computingenvironments. For example, the present invention may be implemented on amulti-nodal system (e.g., LAN, SAN, WAN, MAN) or networking system(e.g., Internet, WWW). All programming and data related thereto arestored in computer memory, static or dynamic, and may be retrieved bythe user in any of: conventional computer storage and display (i.e.,CRT). The programming of the present invention may be implemented by oneof skill in the art of object-oriented and network programming.

1. A method for maintaining consistency, in a Storage Area Network(SAN), of a local copy of a remote file system sub-tree obtained from aremote source; said method comprising: a. mapping directory structure ofsaid remote file system sub-tree to a remote container; said remotecontainer attached to said SAN; b. representing each remote objectcomprising said remote file system sub-tree as a local object componentof said remote container; c. labeling each of said local objects withattributes associated with said represented remote object; d. storingmetadata describing each of said local objects in a metadata server; e.associating a consistency policy with each of said local objects in saidremote container; said policy defining conditions for checking freshnessof said labeled attributes; and f. updating said local object componentsof remote container in accordance with said consistency policy.
 2. Amethod for maintaining consistency, in a Storage Area Network (SAN), asper claim 1, wherein said local objects are hierarchically structuredwithin said remote container in a manner corresponding to hierarchicalstructure of said remote objects comprising said remote file systemsub-tree.
 3. A method for maintaining consistency, in a Storage AreaNetwork (SAN), as per claim 1, wherein said local and remote objectsboth represent either of: a directory or a file.
 4. A method formaintaining consistency, in a Storage Area Network (SAN), as per claim3, wherein said local object is comprised of at least: a directory entryand directory attributes; if said local object represents a directory;and at least: file content and file attributes; if said local objectrepresents a file.
 5. A method for maintaining consistency, in a StorageArea Network (SAN), as per claim 4, wherein said metadata is comprisedof at least a local object attribute; either of: a content storageattribute, if said local object is a file; or at least one directoryentry, if said local object is a directory; and any of: a consistencyguarantee.
 6. A method for maintaining consistency, in a Storage AreaNetwork (SAN), as per claim 5, wherein representing of said remoteobject as a local object comprises: a. storing said file content at aphysical location specified by said content storage attribute; if saidlocal object represents a file; and b. populating said local object withsaid directory entries; if said local object represents a directory. 7.A method for maintaining consistency, in a Storage Area Network (SAN),as per claim 5, wherein said freshness checking comprises: a. triggeringa check of said labeled attributes associated with said local object; b.checking for the existence of said metadata defining a consistencyguarantee for said local object; c. fetching from said remote source,attributes associated with said remote object represented by said localobject, based on results of said checking step; d. creating aconsistency guarantee for said local object, based on results of saidfetching step; and e. merging said fetched attributes with saidattributes associated with said local object.
 8. A method formaintaining consistency, in a Storage Area Network (SAN), as per claim7, wherein said check of labeled attributes is triggered by either of:synchronous or asynchronous methods.
 9. A method for maintainingconsistency, in a Storage Area Network (SAN), as per claim 7, whereinsaid fetched attributes are accompanied by either of: an explicit or animplicit consistency guarantee.
 10. A method for maintainingconsistency, in a Storage Area Network (SAN), as per claim 9, whereinsaid implicit consistency guarantee exists for a pre-determined periodof time.
 11. A method for maintaining consistency, in a Storage AreaNetwork (SAN), as per claim 9, wherein said explicit consistencyguarantee exists until notification is received from said remote source.12. A method for maintaining consistency in a Storage Area Network(SAN), as per claim 7, wherein said freshness checking further comprisesdetermining validity of said local object content by comparing saidfetched attributes with said labeled attributes, based on results ofsaid checking step; said comparison validating said local object if saidlabeled attributes match attributes of said local content, elseinvalidating said local object content.
 13. A method for maintainingconsistency in a Storage Area Network (SAN), as per claim 7, whereinsaid local object is updated with modifications made to a remote objectimported from said remote source.
 14. A method for maintainingconsistency in a Storage Area Network (SAN), as per claim 7, whereinsaid remote object is updated with modifications made to a local objectexported to said remote source.
 15. A system for updating, storing, andaccessing, in a Storage Area Network (SAN), a local copy of file systemdata from a remote source; said system comprising: a. a clientinterface, connected to a SAN client, sending to and receiving from saidSAN client, metadata and a request for access to said local copy; b. aremote container module connected to said client interface; said remotecontainer module managing consistency of said local copy and processingsaid request; c. a cache manager connected to said remote containermodule managing and allocating storage space for said local copy; d. areplica manager connected to said remote container module; said replicamanager maintaining consistency of said local copy if validation isasynchronous type; e. a distributed file manager connected to saidremote container module; said distributed file manager updating saidlocal copy by obtaining said remote source file system data; and f. aconsistency policy maintenance framework connected to said remotecontainer module, said distributed file manager, said replica manager,and said cache manager, said consistency policy maintenance frameworkstoring conditions governing consistency management of the local copy.16. A system for updating, storing, and accessing, in a Storage AreaNetwork (SAN), as per claim 15, wherein said consistency policymaintenance framework stores conditions comprising at least: validationpolicy and type for said local copy and a policy on storage spacereplacement.
 17. A system for updating, storing, and accessing, in aStorage Area Network (SAN), as per claim 16, wherein said a remotecontainer module processes said access request by determining whetheraccess is allowed to said local copy for which said access request wasreceived.
 18. A system for updating, storing, and accessing, in aStorage Area Network (SAN), as per claim 17, wherein said determinationis made by checking metadata to ensure existence of said local copy andif said local copy exists, verifying the validity of said local copy.19. A system for updating, storing, and accessing, in a Storage AreaNetwork (SAN), as per claim 17, wherein said remote container moduleverifies the validity of said local copy by consulting said consistencypolicy maintenance framework to obtain said validation policy.
 20. Asystem for updating, storing, and accessing, in a Storage Area Network(SAN), as per claim 18, wherein said access request is granted if saidlocal copy exists and is valid; said access request denied otherwise.21. A system for updating, storing, and accessing, in a Storage AreaNetwork (SAN), as per claim 19, wherein said validation policy compriseschecking for either of: the existence consistency guarantee or proofthat said remote file system data has been imported within a time boundspecified by said validation policy.
 22. A system for updating, storing,and accessing, in a Storage Area Network (SAN), as per claim 20, whereinsaid remote container module manages consistency of said local copy bydelegating to said distributed file manager, the task of importing saidremote file system data to update said local copy if said access requestis denied.
 23. A system for updating, storing, and accessing, in aStorage Area Network (SAN), as per claim 22, wherein said distributedfile manager imports said remote file system data in accordance with atleast a local copy update policy obtained from said consistency policymaintenance framework.
 24. A system for updating, storing, andaccessing, in a Storage Area Network (SAN), as per claim 22, whereinsaid remote container module manages consistency of said local copy bydelegating to said cache manager, the task of allocating storage spacefor said imported remote file system data.
 25. A system for updating,storing, and accessing, in a Storage Area Network (SAN), as per claim24, wherein said cache manager consults said consistency policymaintenance framework to obtain a policy on storage space replacement inservice of said storage space allocation task.